I am using a Cubietruck. I have Armbian Jessie on a bootable uSD card. I have Arch Linux ARM on my SATA drive.

I want to be able to tell the uSD card to boot from /dev/sda1 instead.

The whole boot process is a mystery to me. If I change boot.cmd, do I have to "activate" it somehow?

Why doesn't /boot contain uEnv.txt by default?If I create /boot/uEnv.txt, will it be used automatically?Will it be used in preference to boot.cmd?Do I need to install any extra software in order to get uEnv.txt to work?